The microbiota is the entire community of microbes in a particular niche, such as the colon. The goal of the microbiota projects is to identify targets for cancer diagnosis or intervention through research on the microbiota of feces that has been prospectively and cost effectively collected from a general population of many thousands with essential covariate data and good ascertainment of cancer events. Compared to healthy people, we have identified significant alterations of the fecal microbiota in survivors of Hodgkin lymphoma and in people newly diagnosed with colorectal pre-cancer or postmenopausal breast cancer. Current efforts are to validate these associations and to examine whether the cancer-microbe associations are related to alterations immunity or inflammation, in levels or metabolism of hormones, or in small metabolites produced by the microbes or host.